As opposed to mono audio recording, stereo audio recording and playback has been used to provide a richer audible user experience. In stereo audio recording, a music signal is mixed into “left” and “right” signals, so that when played through a pair of speakers spaced a nominal distance apart, the listener perceives a sound field, that offers the illusion of instruments placed in actual locations on a virtual stage. In human hearing, the brain determines the apparent location of an event, like the apparent placement of individual instruments, based upon the arrival time, subtle frequency response differences and intensity of the sound waves that reach each of the listener's ears. The quality and realism of a stereo audio image experienced by the listener is dependent upon the design and quality of the speakers used for playback, and the location of the speakers, relative to the listener.
In recent years, several software programs have been used to enhance and add greater three dimensional effects to ordinary stereo, through computations and manipulations of the individual sound signals, in some cases using digital signal processing techniques. However, regardless of the method used to create or enhance a stereo audio signal, the spacing of the speakers, relative to the listener, remains a major factor in the quality of the stereo image experienced by the listener.
For many handheld type devices, like wireless communication devices, there is a trend toward smaller devices. Smaller devices are easier to carry on one's self, where the smaller the device, the greater the number of options for the location where the user can store the device (i.e a pocket, a belt clip, a small hand bag, etc.). However, as the device size decreases, so does the volume and surface area of the device, within and upon which the components can be located, which the user uses to interface with the device. For example, displays, microphones, speakers and keypads have external interfaces that are generally located at various positions around the external surface of the device, where they are conveniently accessible by the user, and are coupled to corresponding components internal to the device.
When adding stereo playback capability to a wireless communication device or other hand held portable electronic device, the placement of the right and left speaker is constrained by the overall size of the device, resulting in a relatively close spacing of the two speaker elements. For a wireless communication device the problem is further frustrated by the typical aspect ratio of hand held devices, which are narrower in the horizontal direction and taller in the vertical direction. This is due to the fact, that the speakers need to be generally separated in the horizontal direction, which corresponds to the listener's ears, which are similarly displaced from one another in a generally horizontal direction.
Still further the depth of the device, or distance between the front facing of the device and the back facing of the device, is relatively shallow, as compared to both the height and the width of the device. As a result, in prior hand held devices, the speakers or transducers, have been oriented with the plane of their front facing parallel to the front facing of the device. Traditionally, in these instances, the sound from the speakers has been ported directly out of the device in a direction of travel that is generally perpendicular to the front facing of the speakers. However given the necessary diameter of many speakers, this has resulted in the output ports for the speakers being further limited in their horizontal displacement. For example, a 40 mm wide cellular telephone, incorporating a pair of 18 mm diameter transducers, would have their acoustic centers separated by approximately 19 mm. Such a distance would limit the quality of a stereo image.
The present inventors have recognized, that if instead of directly porting the sound from the speakers in a direction that is perpendicular to the front facing of the device, the sound waves produced by the speakers are directed away from the front or back facing of the device, toward alternative ones of the two opposing side facings of the device. In this way the location of the ports, which are now located proximate alternative side edges of the device, becomes the apparent source of the respective audio signals, thereby maximizing the apparent relative horizontal displacement of the right and left audio sources.